This invention is generally related to methods and compositions for treating well bores in subterranean formations, and more particularly to oil-based drilling fluids and additives suitable for such fluids to improve their suspension characteristics.
A drilling fluid, or “mud” which a drilling fluid is also often called, is a specially designed fluid that is circulated in a well bore as the well bore is being drilled to facilitate the drilling operation. The various functions of a drilling fluid include removing drill cuttings from the well bore, cooling and lubricating the drill bit, aiding in support of the drill pipe and drill bit, and providing a hydrostatic head to maintain the integrity of the well bore walls and prevent well blowouts.
Specific drilling fluid systems are selected to optimize a drilling operation in accordance with the characteristics of a particular geological formation. A drilling fluid typically comprises water and/or oil, synthetic oil, or other synthetic material or fluid as a base fluid, with solids in suspension. A non-aqueous based drilling fluid typically contains oil or a synthetic fluid as a continuous phase and may also contain water dispersed in the continuous phase by emulsification so that there is no distinct layer of water in the fluid. Such dispersed water in oil is generally referred to as an invert emulsion or water-in-oil emulsion. A number of additives may be included in such oil based drilling fluids and invert emulsions to improve certain properties of the fluid. Such additives may include, for example, emulsifiers, weighting agents, fluid-loss additives or fluid-loss control agents, viscosifiers or viscosity control agents, and alkali.
An essential criterion for assessing the utility of a fluid as a drilling fluid or as a well service fluid may include the fluid's rheological parameters, particularly under simulated drilling and well bore conditions. For use as a drilling fluid, or as a fluid for servicing a well, a fluid generally should be capable of maintaining certain viscosities suitable for drilling and circulation in the well bore. Preferably, a drilling fluid will be sufficiently viscous to be capable of supporting and carrying the well drill cuttings to the surface without being so viscous as to interfere with the drilling operation. Moreover, a drilling fluid must be sufficiently viscous to be able to suspend barite and other weighting agents. However, increased viscosity can result in problematic sticking of the drill string, and increased circulating pressures can contribute to lost circulation problems.
Commonly, additives are included in the drilling fluid to increase the viscosity or carrying capacity of the fluid. These additives may include solids, including fine or colloidal solids, that may add density to the weight of the fluid while at the same time increasing its viscosity. However, the addition of solids may be detrimental as a fluid may exhibit better rheological properties in a clean state. Faster drilling rates may also be achieved by employing low-solids or solids-free fluids in some cases. When porous hydrocarbon-producing formations are drilled, the size and composition of suspended solids may be regulated to provide optimal bridging and minimize invasion. Calcium carbonate may be used in these situations as it may generally be acid-soluble, and thus provides a removable filter cake. Other clay-based solids commonly used in drilling fluids may be less desirable since they do not provide the same advantage.
Organophilic clays may commonly be used as the viscosity-building agent in oil-based muds. These generally comprise amine-treated bentonite or hectorite clay, which may typically require vigorous agitation in the fluid to reach a fully active state and provide an increase in viscosity. Time pressures may demand that fluids be prepared quickly for shipment to the drilling operation. As a result, inadequate shear and over-treatment may occur when using organophilic clays.
In addition, small particles of solid matter may ultimately be too small to be effectively removed from the system by surface processing equipment during and after completion of the well bore. As a result, fluids which do not contain organophilic clays may offer advantages in several aspects of drilling.